REVIEWING AND REVISING THE PROTEROZOIC HISTORY OF THE FIVE POINTS SHEAR ZONE: A MULTIPLY DEFORMED PALEOPROTEROZOIC SHEAR THAT FORMED PRIOR TO 1.67 GA

The Five Points shear zone (FPSZ) is located in the Wet Mountains, CO, just west of Cañon City. It strikes north-south (mean attitude 338°, 58°NE), is on strike with the strike-slip portion of the Battle Lake Fault Zone (BLFZ) in the Sierra Madre, and had previously been interpreted to form ca. 1.4 Ga (e.g. Jones, 2005, Jones et al., 2010b), based on mineral growth ages and cross-cutting Mesoproterozoic dikes. During this study, however, we found that at least two of the crosscutting mafic dikes crystallized before ~1.67 Ga, implying an earlier deformational history for the FPSZ than previously thought. The FPSZ’s earliest, dominant fabric was formed between 1739-1660 Ma (taking age uncertainties into account). The Five Points Gulch shear zone (FPSZ) cuts a folded package of intercalated metasedimentary and metavolcanic rocks (Bickford and Boardman, 1984; Bickford et al., 1989a; Cochrane, 2013), which formed at 1733±6 Ma (Cubrich, 2017). Mafic dikes cut the dominant fabric of the shear zone. At least two of these dikes are ≥1.67 Ga based on U-Pb zircon date of 1672±12 Ma and metamorphic titanite dates of 1631±15 Ma and 1662±18 Ma (Cubrich 2017). Thus, the dominant fabric of the FPSZ must have formed prior to 1.67 Ga. The FPSZ is a candidate for the Proterozoic structure that accommodated large scale transform faulting from 1.68 to 1.55 Ga, which brought a western lithospheric block nearly 200 km north relative to basement in east-central CO. Because the dominant period of deformation occurred prior to 1.67 Ga, with only minor reactivation, we suggest that ca 1.4 Ga events along the zone were dominated by heating with little deformation.